There are several circuits and topologies of battery chargers for charging rechargeable batteries. In the case of a relatively large capacity battery of the type used in cordless power tools, the battery charger involves an AC-to-DC switching converter that receives AC wall power and that outputs a conditioned DC voltage supply. An efficient DC-to-DC switching converter is powered by the DC voltage supply. This DC-to-DC switching converter handles charging the battery. This same basic topology and circuit partitioning is used for other types of rechargeable batteries and for batteries of different capacities. For example in the case of an auxiliary solar-powered battery charger for charging the battery in a cellular telephone, a first power stage having solar cells outputs a supply voltage. The amount of current output by the solar cells varies considerably, depending on the amount of light incident on the solar cells. This supply voltage is supplied to the cellular telephone via a connector such as a USB (Universal Serial Bus) connector. Within the cellular telephone is a charger circuit that is especially adapted to charge the particular type of rechargeable battery that is part of the cellular telephone. The charger circuit receives power via the USB connector, and conditions it and supplies power to the battery at the appropriate voltage and current, in accordance with the proper charging profile for the type of rechargeable battery within the cellular telephone. Due to this circuit partitioning, the battery within the cellular telephone can alternatively be charged via the USB connector by a large number of devices that output a supply voltage to a USB plug, such as for example a car adapter, an auxiliary battery pack, or an AC-to-DC wall adapter. In each case, the second power stage within the cellular telephone is specially adapted to charge the battery in accordance with the proper charging profile. This two-stage partitioning and topology is desirable in that the second stage is made to tailor to the needs of the particular rechargeable battery. If the current demands are low enough, and if the supply voltage supplied to the second stage is appropriate, then the two-stage circuit partitioning is even more desirable because linear power conversion can be employed in the second stage, thereby reducing cost and complexity and size of the overall battery charger circuit. Depending on the particular type of battery charger, the connection and connector between the two stages may be a USB connector (USB plug and USB socket), a barrel connector (barrel plug and socket), a battery pack stand, a docking connector, or other type of connector.